Re: [PATCH v6 2/6] arm64: ptrace: allow tracer to skip a system call

Kees Cook
2014-08-21T17:08:50Z

On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro
<takahiro.akashi@linaro.org> wrote:
> If tracer specifies -1 as a syscall number, this traced system call should
> be skipped with a value in x0 used as a return value.
> This patch enables this semantics, but there is a restriction here:
>
> when syscall(-1) is issued by user, tracer cannot skip this system call
> and modify a return value at syscall entry.
>
> In order to ease this flavor, we need to treat whatever value in x0 as
> a return value, but this might result in a bogus value being returned,
> especially when tracer doesn't do anything at this syscall.
> So we always return ENOSYS instead, while we have another chance to change
> a return value at syscall exit.
>
> Please also note:
> * syscall entry tracing and syscall exit tracing (ftrace tracepoint and
> audit) are always executed, if enabled, even when skipping a system call
> (that is, -1).
> In this way, we can avoid a potential bug where audit_syscall_entry()
> might be called without audit_syscall_exit() at the previous system call
> being called, that would cause OOPs in audit_syscall_entry().
>
> * syscallno may also be set to -1 if a fatal signal (SIGKILL) is detected
> in tracehook_report_syscall_entry(), but since a value set to x0 (ENOSYS)
> is not used in this case, we may neglect the case.
>
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
> ---
> arch/arm64/include/asm/ptrace.h | 8 ++++++++
> arch/arm64/kernel/entry.S | 4 ++++
> arch/arm64/kernel/ptrace.c | 20 ++++++++++++++++++++
> 3 files changed, 32 insertions(+)
>
> diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
> index 501000f..a58cf62 100644
> --- a/arch/arm64/include/asm/ptrace.h
> +++ b/arch/arm64/include/asm/ptrace.h
> @@ -65,6 +65,14 @@
> #define COMPAT_PT_TEXT_ADDR 0x10000
> #define COMPAT_PT_DATA_ADDR 0x10004
> #define COMPAT_PT_TEXT_END_ADDR 0x10008
> +
> +/*
> + * used to skip a system call when tracer changes its number to -1
> + * with ptrace(PTRACE_SET_SYSCALL)
> + */
> +#define RET_SKIP_SYSCALL -1
> +#define IS_SKIP_SYSCALL(no) ((int)(no & 0xffffffff) == -1)
> +
> #ifndef __ASSEMBLY__
>
> /* sizeof(struct user) for AArch32 */
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index f0b5e51..fdd6eae 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -25,6 +25,7 @@
> #include <asm/asm-offsets.h>
> #include <asm/errno.h>
> #include <asm/esr.h>
> +#include <asm/ptrace.h>
> #include <asm/thread_info.h>
> #include <asm/unistd.h>
>
> @@ -671,6 +672,8 @@ ENDPROC(el0_svc)
> __sys_trace:
> mov x0, sp
> bl syscall_trace_enter
> + cmp w0, #RET_SKIP_SYSCALL // skip syscall?
> + b.eq __sys_trace_return_skipped
> adr lr, __sys_trace_return // return address
> uxtw scno, w0 // syscall number (possibly new)
> mov x1, sp // pointer to regs
> @@ -685,6 +688,7 @@ __sys_trace:
>
> __sys_trace_return:
> str x0, [sp] // save returned x0
> +__sys_trace_return_skipped: // x0 already in regs[0]
> mov x0, sp
> bl syscall_trace_exit
> b ret_to_user
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index 8876049..c54dbcc 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -1121,9 +1121,29 @@ static void tracehook_report_syscall(struct pt_regs *regs,
>
> asmlinkage int syscall_trace_enter(struct pt_regs *regs)
> {
> + unsigned int saved_syscallno = regs->syscallno;
> +
> if (test_thread_flag(TIF_SYSCALL_TRACE))
> tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
>
> + if (IS_SKIP_SYSCALL(regs->syscallno)) {
> + /*
> + * RESTRICTION: we can't modify a return value of user
> + * issued syscall(-1) here. In order to ease this flavor,
> + * we need to treat whatever value in x0 as a return value,
> + * but this might result in a bogus value being returned.
> + */
> + /*
> + * NOTE: syscallno may also be set to -1 if fatal signal is
> + * detected in tracehook_report_syscall_entry(), but since
> + * a value set to x0 here is not used in this case, we may
> + * neglect the case.
> + */
> + if (!test_thread_flag(TIF_SYSCALL_TRACE) ||
> + (IS_SKIP_SYSCALL(saved_syscallno)))
> + regs->regs[0] = -ENOSYS;
> + }
> +

I don't have a runtime environment yet for arm64, so I can't test this
directly myself, so I'm just trying to eyeball this. :)

Once the seccomp logic is added here, I don't think using -2 as a
special value will work. Doesn't this mean the Oops is possible by the
user issuing a "-2" syscall? As in, if TIF_SYSCALL_WORK is set, and
the user passed -2 as the syscall, audit will be called only on entry,
and then skipped on exit?

-Kees

> if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
> trace_sys_enter(regs, regs->syscallno);
>
> --
> 1.7.9.5
>

--
Kees Cook
Chrome OS Security